Exploring Piston And Ring Technologies

A Better Understanding Of Piston And Ring Technologies Can Lead To A More Potent Engine Build

Eric English

August 23, 2010

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Tech | Piston And Ring Technologies
Whether you're considering new cars, or the high-performance aftermarket, engines are being built with more power than ever before. Items like power adders, multiple camshafts, and EFI tend to get all the glory, but it's true that virtually no stone has been left unturned in the quest for improved efficiency and power. Buried beneath all the flash, pistons and rings play an obviously critical, though somewhat overlooked role in the success of any engine. Influencing horsepower and durability, piston and ring technology has evolved through the years, and we felt it pertinent to review current thinking.

In reality, today's pistons serve the same purpose they have since the dawn of the internal combustion engine-converting the energy of combustion to mechanical energy. From there on however, it's a whole new world, with engineering and manufacturing capabilities at all time highs. Clean slate piston designs incorporate computer 3D modeling and Finite Element Analysis (FEA), helping identify areas of stress before a prototype is ever manufactured. CNC machining results in incredible precision, while space age coatings offer properties that were unheard of when the cars we love were first built.

In reality, the materials used for manufacturing pistons are much as they have been for years, with cast and forged aluminum being the real world choices. Castings are a perfect fit for many OEM applications due to low cost, and low thermal expansion, but their durability is not adequate for more demanding high-performance environs. Hypereutectic castings are a notable exception, and can be a sensible choice in a variety of performance endeavors (see our sidebar, Hyperwhat?). Billet aluminum is sometimes used in high-end racing pistons, but this aside; the most durable slugs for serious high performance are forgings. In fact, consider them mandatory in the world of high boost and big nitrous hits.

JE's Randy Gillis explained that the aluminum alloys used in today's forgings were developed before WWII by Rolls Royce, for use in aviation. These alloys are 2618 and 4032, which have different characteristics which are important to understand. The 2618 is the most durable in extreme environs, but with a higher thermal expansion rate than 4032, and requires greater piston to cylinder wall clearances when cold. The 4032 is made from material with higher silicon content, enabling tighter cold cylinder clearances that are particularly desirable to the OEMs due to emissions. However, the higher silicon content of 4032 (roughly 12 percent) makes it less ductile (more brittle) than 2618, thus it's ultimately not as friendly to punishing abuse.

The Science Of Sealing-Today's Rings
One of the biggest changes in piston design over the years is really all about rings. As a quick review, rings serve three functions in an engine: 1) they keep combustion in the chamber, 2) they help transfer heat from the piston to the cylinder wall, and 3) they keep oil where it belongs. In the heyday of the muscle car, common ring thicknesses were 5/64-inch for the top two rings, and 3/16-inch for the oil ring. If you haven't built an engine for a while, you may be surprised at the changes, because thin is in! As CP's Ed Urcis told us, 1/16-inch is the new 5/64-inch, while many applications are much thinner. Take for instance the GM LS and Ford Modular V-8s, which typically come from the factory with a metric sized 1.5mm, 1.5mm, 3.0mm package. Racing applications are often much thinner still, and make use of strategies such as gas porting, but the point is obvious-thinner rings develop less friction, and perhaps surprisingly, seal better as they conform more easily to the cylinder bore.

So how thin is thin? We asked several manufacturers about the thinnest ring set they would recommend for what we view as our broadest readership-those with hot street cars that see occasional track use. While there wasn't unanimous agreement, everyone was comfortable with the metric 1.5mm, 1.5mm, 3.0mm combo mentioned previously. Many were good with thinner 1.2mm or 0.043-inch top rings, including Total Seal's Kevin Studaker, who mentioned the edgy possibility of a 0.043mm, 0.043mm, 3.0mm package for normally aspirated and modest boost situations. Studaker also offered that when heading above 20 psi of boost, a bit thicker combination such as 1/16-inch, 0.043-inch, 3/16-inch is likely to result in better sealing and durability over the long haul. Of course, since pistons must have corresponding sized grooves, much of this will be dictated by whether you plan your next engine by considering your ring package first. As will become evident as you read this story, rings really are the prudent place to start.

Budgets will also dictate. Not only are thinner rings typically more expensive since they tend to use more costly materials, what you consider to be an ultimate ring package may not be available with an off the shelf piston. The necessary custom pistons can easily cost twice that of a modest forged catalog part, and often much more depending on options. Premium ring sets that include thin stainless steel top rings and Napier second rings are in the range of $300 per set, whereas a similar dimension ductile iron/non-Napier set for the same engine is in the $100 range. The financial equation was emphasized by Probe's Shawn Mendenhall when explaining why Probe's economical FPS forged piston line still uses a conventional 5/64-inch, 5/64-inch, 3/16-inch ring pack. Enthusiasts on a budget seem to appreciate the fact that such traditional size rings are noticeably less expensive than thinner sets, even when comparing like materials. Consistent with the philosophy of offering a wide range of products and prices, Probe's more expensive SRS forged line comes with a more modern 1/16-inch, 1/16-inch, 3/16-inch combination.

But there is more to a modern ring set than just thickness. There are new materials, new designs, and to be certain, different schools of thought. In the old days, the top two rings were typically cast-iron, or perhaps a better ductile iron for the top ring with a chrome or flame-sprayed molybdenum coating. Today, ductile iron is the standard bearer for performance top rings, and in all likelihood, with a plasma-sprayed moly coating-thus the term plasma-moly. A big step up for top ring material is stainless steel, particularly as the thickness narrows. Depending on the manufacturer, stainless will be chrome nitrided or plasma-moly, and in some high-end racing applications, can even be tungsten or titanium nitrided.

Second rings have long been cast-iron since the heat and abuse in this location is much less than the top ring. That's pretty well the case today, and yet along with thickness, perhaps the biggest change in the second position is the growing use of Napier rings-which dovetails with current thinking that the second ring is really more about oil control than compression. Napier rings are seen in OEM applications, and every aftermarket manufacturer we spoke to advocated their use whenever possible. Being a better oil controller than the typical reverse torsional taper face second, a Napier is designed to put less pressure on the cylinder wall, and also allows for a lower tension oil ring-a double whammy friction reduction!

Oil rings are still a three-piece affair, with an expander located between two rail rings. You might be surprised to learn that the majority of friction in a ring package is right here, sometimes on the order of 60-70 percent depending on oil ring tension. Is there a way to free up power and efficiency here? Yes, but going to a "low tension" oil ring isn't necessarily the answer, because the label doesn't apply equally across the board. For example, a 3/16-inch low-tension ring will certainly have less tension than a 3/16-inch standard-tension ring, but that 3/16-inch low-tension will actually have more tension than a 3mm standard-tension ring. Basically you can only compare terminology within the same sized product. Akerly and Childs' Ray Akerly explained that if when comparing a 3/16-inch and a 3mm oil ring, each exerted the same amount of pressure on the cylinder wall, the thinner 3mm ring would seal better since it is better able to conform to inevitable bore distortion. Total Seal's Studaker added that for a hot street engine with a good finish hone and all else in order, 15-16 pounds of pressure on the wall is adequate, which equates to a low-tension 3/16-inch, or, believe it or not, a high-tension 3mm. As a frame of reference, a standard-tension 3/16-inch oil ring is in the range of 20-24 pounds of pressure.

Gapless rings are considered by some as the ultimate top ring, and are sometimes used in the second position as well. They've won plenty of fans, while many others remain solidly in the traditional camp. The idea is that without any gap at all, you eliminate virtually all possibility of combustion getting past the top ring, resulting in more power. Likewise as the cylinder wears over time, there is no end gap to increase in size. It's a concept that would appear to have merit, as the goal in a conventional ring is to be as close to zero gap as possible at operating temps, while avoiding the ends butting together. Perhaps a test conducted by our sibling magazine Engine Masters Challenge, best illustrates the potential of a gapless top ring. In the test of a 450-horse Chevy (sorry guys), Total Seal's gapless top rings made 10 more peak horsepower when compared to a set of file fit rings-the latter being no slouch in their own right!

Back in the world of conventional rings, much more consistent opinions were offered when the subject of the second ring gap was discussed. In years past, it was common to run a tighter gap on the second ring, compared to the top ring, again trying to achieve a near zero gap. This could be done since the second ring is subject to less heat, and thus doesn't expand as much as the top ring. That concept has largely been replaced with the idea that the second ring gap should be bigger-perhaps on the order of 1.25 times the top gap, to allow any gasses which slip past the top ring a way out. Trapped gasses between the first two rings will tend to unload the top ring and diminish its seal, which is also why many newer pistons include an accumulator groove between the first and second ring grooves-it acts as a reservoir for blow-by.

Dangerous Intersections?
In the early days of affordable stroker kits, there was much talk about combinations in which the oil ring would intersect the wristpin, potentially resulting in increased oil consumption. In the Ford world, this discussion largely revolved around short deck stroker small-blocks, where the 331 combination that didn't require such a ring package, was at one time considered more street friendly than the 347 stroker, which did. That debate has faded significantly as the modern oil ring rail support has been perfected-essentially an 0.030-inch reinforcing ring that sits at the bottom of the oil ring groove-keeping the oil ring intact and in place. JE's Randy Gillis believes piston stability is the determining factor regarding oil control issues, and hasn't found the intersecting ring/pin to be an issue in current testing. Truth is, there are lots of situations beyond the 331/347 where there is potential for an intersecting oil ring and pin, so lots of effort has gone into making it work. The issue is common on stroker Clevelands and 385-series big-blocks since the top ring is lower on the piston due to the canted valves, as well as Modular strokers. High boost applications can have the same issue because the ring pack is typically moved down on a custom piston for such situations.

To Coat, Or Not...
Aerospace coatings began to enter the automotive world in the late 1980s, with pistons identified as prime candidates. As we spoke with various people in the industry, we found some debate surrounding when piston coatings should be used. It seemed to boil down to three schools of thought: 1) a properly prepared engine shouldn't need coatings, 2) there are situations where coatings are advantageous, and 3) coatings are nearly universally advantageous. We won't weigh in on the discussion, but would encourage you to take up the issue with your particular engine builders. Trust their experience more than your book knowledge.

What we will do is describe what coatings are designed to do. Two types of coatings are applied to pistons. One is a "dry film" anti-scuff skirt coating that is some kind of proprietary blend of molybdenum, graphite, Teflon, and epoxy, which is screened onto the piston skirt and heat cured. The other common piston coating is a metallic ceramic thermal barrier that is applied to the piston top. The idea here is that deflecting heat from combustion helps the piston to live and keeps heat in the chamber where it may result in greater power. Forced induction would seem to be an application ripe for thermal coatings, but the real benefit to both coatings involves increased durability first, and the potential for additional power as second.

Several piston companies offer skirt coatings as standard treatment on particular product lines, while nearly all others can provide coatings as an option. If you're interested in coating pistons you've already purchased, companies like Calico Coatings can do the job. Calico quoted the following prices: $21 per piston for skirt coatings, $26 per piston for thermal top coatings, and $39 per piston for both treatments.

Apples To Apples
Ring thickness measurements may be given in fractions of inches, millimeters, or decimal-inch form. In an effort to help illustrate how thin rings have become, we've converted all the ring sizes we discussed with manufacturers into a common decimal-inch form. Remember, these are rings which could be considered street or street/track worthy depending on your particular priorities. It's worth noting that much thinner rings exist in automotive racing applications.

3/16-inch = 0.1875-inch

3mm = 0.118-inch

5/64-inch = 0.078-inch

1/16-inch = 0.0625-inch

1.5mm = 0.059-inch

1.2mm = 0.047-inch

0.043-inch = 0.043-inch

Did You Know?
In our conversation with JE's Randy Gillis, he mentioned several basics of piston design we thought worth passing along. When looking down on a piston, the shape is slightly oval rather than perfectly round, though you won't detect it with your eyes. Gillis emphasized that aluminum acts like a heat sink, and where there is more aluminum, more heat will be absorbed. Such areas will expand more when heated, and since the supporting structure for the wristpins is one of the beefiest areas of a piston, the piston is 0.004-0.008-inch smaller in this dimension than it is at locations 90-degrees from the pins. As well, the side profile of a piston is not perfectly straight. Again, since there is more aluminum toward the top of the piston than there is in the skirt, the top will be 0.030- to 0.035-inch smaller in diameter to allow for more expansion. Speaking of temps, Gillis mentioned that the top of a piston can easily be 500 degrees Fahrenheit, whereas the skirt will typically run around 200 degrees Fahrenheit. As for rings, did you know that rings rotate around the piston during operation? It's true, otherwise the ring end gap would groove the cylinder wall. Also, the face of the top ring is described as a barrel-face, or rounded, which has been found to provide a better seal than the more square shouldered top rings of days gone by. As for the second ring, they have a taper to the face-the bottom of the ring touching the cylinder wall while the top is slightly narrower, resulting in better oil scraping action.

Hyper what?
You've no doubt seen the term "hypereutectic" when it comes to piston composition, but what does the term actually mean? Keith Black's Scott Sulprizio explained that a eutectic alloy results when silicon is mixed with aluminum to the point of maximum saturation or absorption, meaning any additional silicon would fall out of suspension during creation of the alloy. A 12 percent mix of silicon is the eutectic point in an aluminum alloy, with a higher silicon content resulting in a hypereutectic product, and a lower silicon content technically being hypoeutectic.

Of course aluminum must be melted to mix silicon and other additives which comprise an alloy, but heating it to four times the melting temp of aluminum will also melt the silicon into the solution when creating a hypereutectic. When cooled, these silicon particles reform throughout the piston body, but have much different characteristics than before they were melted. "They're extremely hard, almost ceramic-like," says Scott. The result is a much tougher piston that has excellent thermal properties (low expansion and low heat absorption), and resists wear to a much higher degree than a standard casting. Good as hypereutectic material is, Scott went on to offer that "there isn't a perfect alloy that does it all. Hypereutectics keep more heat in the combustion chamber and have nice tight cold tolerances, but the flip side to adding more silicon is that you lose ductility." In other words, the product is more brittle than a low silicon alloy.

Pistons manufactured from forged material, where tremendous pressure is applied to create a denser crystalline structure and alleviate porosity, are considered more durable than any casting regardless of material, so when does one decide that they've exceeded the limits of a hypereutectic? That's hard to say, though Scott feels it's all about engine management. "As horsepower goes up, the engine management becomes way more important, with spot-on air/fuel ratios and avoiding detonation becoming absolute keys to piston survival. Hypereutectics are well suited to 400- to 500-horsepower applications when the tune is sharp, and the OEM's big-dollar computers are able to do this very well." What about the shade tree mechanic with his box-stock carburetor? It's easy to predict that his engine management skills won't be as sharp.

Federal Mogul's Scott Gabrielson was a bit more specific when trying to recommend a point where his Speed Pro/Sealed Power customers should consider moving from hyper-eutectics to a forging. He agrees that hypereutectics are plenty strong for many high-performance engines, articulating that they're physically stronger than a 4032 forging under normal operating temperatures. The caveat is that when something goes awry, the failure is dramatic, and Scott offered that, "hypereutectics aren't friendly to detonation." For this reason, a couple of his trip points to going forged would be if boost exceeds 8 psi, or if nitrous use of more than 150 horses is in the works. For the sake of comparison, hypereutectic alloy has 16-18 percent silicon content, 4032 has 11 percent silicon, and 2618 has about 1 percent silicon.